The release of nitrogen and phosphorous compounds into the environment has become one of the most pressing environmental hazards. The release is generally of no significance in low concentrations due to natural presence. However, they are released in large quantities especially in industrial, domesticated farm, aquaculture or forestry agriculture areas and cause eutrophication of water recipients, first producing algal blooms and thereafter oxygen deficiency in natural waters.
Stringent requirements on the processing of biologically produced nutrients and materials are one of the limiting constraints for the establishment of new industrial, food processing, agro- and aquaculture ventures. This is especially true in parts of Scandinavia, and also around the Mediterranean, mid Europe, the US and Canada, parts in Oceania, South East Asia, and several developing countries.
This invention focuses on removal of BOD (biological oxygen demand), nitrogen and phosphorous by biological means. Numerous biological treatment processes have been developed which typically use single or double reactors comprising autotrophic (ammonia and nitrite oxidation) and heterotrophic (aerobic organic oxidation and anoxic denitrification) processes. They are often of a single activated sludge type or fixed bed type, using organic matter in the influent for the removal of nitrogen or/and phosphate (e.g. WO 96/04784, U.S. Pat No. 3,871,999).
The use of activated sludge is cost efficient in large urban waste water treatment plants. However, activated sludge is difficult to control (as in high intensive aquaculture systems with high rates of recirculation and water flow), due to a necessary aggregation into flocs with a subsequent floatation or sedimentation, which are all difficult to control. It puts requirements on large sedimentation or flotation tanks, which in turn lowers the cost efficiency (U.S. Pat. No. 3,849,303, U.S. Pat. No. 5,611,927).
Some of the patented innovations in this field are made up of systems that change the cycles of purification in one or more reactors over time, sometimes called sequenced batch reactor technique (U.S. Pat. No. 4,188,289, U.S. Pat. No. 4,948,510). This means that the microorganisms are subjected to different forms of stress that will lead to loss of growth yield and efficiency of the filters or reactors, due to constraints of metabolic reversals in each cycle and interspecies competition.
Other disclosures use fixed bed bioreactors. Fixed bed reactors for purification as well as any reactor where most of the active biomass is attached as a biological film on an immobilized media, are subject to problems with clogging and requirement of back flushing of the filter media (U.S. Pat. No. 5,081,954). Such back flushing removes the biofilm or parts of the biofilm, creating a lag phase for the regeneration of full capacity of the filter. Furthermore, the uniform distribution of nutrients, oxygen, and carbon through the filter is very difficult to control in fixed bed reactors, where a uniform distribution is actually a prerequisite for an effective process. Numerous examples on patents disclose solutions for an even distribution of water flow in a fixed bed filter, but in practice, it is impossible to fully control the even distribution of bacteria, substrate and electron acceptors as oxygen or nitrate in the media. More importantly, these constraints impair the means of reliable industrial control and optimisation. Fluidised sand bed reactors or sand or fluidised bead filters are used to a large extent in the US (e.g. U.S. Pat. No. 5,792,386). They claim high removal rates of BOD and nitrogen. However, the energy input in these systems is relatively high since they are driven by high-pressure pumps, whereby cost efficiency is lost compared to low head systems, although high pressure systems require small footprint area relative to the internal specific filtration area.
In addition, in flow through systems like waste water treatment plants, where almost all influent inorganic nitrogen is in the form of ammonium, the water has to be nitrified (ammonium is oxidized to nitrite and nitrate) before It is denitrified (nitrite and nitrate is reduced to nitrogen gas). In post denitrification systems the denitrification process is therefore placed after nitrification reactors (U.S. Pat. No. 3,849,303, U.S. Pat. No. 5,611,927). The denitrification requires an easily biodegradable organic substrate while the nitrifying autotrophic bacteria on the other hand, require very low concentrations of biodegradable organic substrate to be able to compete with the heterotrophic bacteria. Therefore, such systems will fail in either the nitrification process or In the denitrification process if not a nearly complete degradation of organic matter precedes the nitrification and an easily biodegradable hydrocarbon is added to the denitrification process.
Alternatively, one may use predenitrification where the denitrification is put as the first reactor constituent in the system (counted from the waste producing process), e.g. an activated sludge plant, and the treated waste water is recirculated back to the denitrification reactor from a subsequent nitrification reactor after the water has been nitrified. In this way the organic matter to be removed is used for the denitrification process as well. However, hydraulic limitations of each subprocess in the loop limit the recycle and therefore limit the maximum nitrogen removal.
In a closed loop system, where only a small part of the water is exchanged with the surrounding environment, the oxidation of organic matter before the denitrification process, like in conventional systems and patent disclosures (WO 96/04784), poses a reduction In the efficiency of biological water treatment, because available organic material that is desirable for the denitrification is lost in the initial oxidation process. High efficiency in the nutrient and organic removal is achieved by organising the biological processes in the energetically and biochemically most efficient sequence. In such a system the natural biodegradable carbon in the production process effluent is used optimally if the denitrification process precedes a heterotrophic oxidation before nitrification, like in the present invention.
It is the sequence of the biological treatment processes relative to the production unit and the inflexibility of the chosen structures that are the major limitations in disclosures for water purification. WO 97/49279 discloses one example where the denitrification is placed in a recycle after the nitrification, and hence there will be no or only limited denitrification if an externally added carbon source is not added. Further, the hydraulic load on the entire treatment process will be unnecessarily high. In another embodiment in the same patent, where the sequence could be argued to be correct (first a denitrification process with a by-pass, followed by a carbon filter and nitrification) the inflexibility of not having a by-pass over the nitrification process will imply a very inefficient use of the nitrification reactor If the nitrite levels are not to become dangerously high to many aquatic organisms. Further, the nitrification process is pursued in a 4″ gravel bed that has the obvious large footprint disadvantage when run in high intensive systems. In WO 96/04784 the nitrification is placed first, which will imply that there will be almost no nitrification as long as there are biodegradable organic matter in the effluent of the production process (fish). The placement of the denitrification process after the nitrification implies as argued that an external carbon source has to be added as well.
The greatest challenge of all in biological water purification processes is developing environments for high efficiency of the nitrification process, which is far more sensitive than denitrification and BOD-removal. Inefficient nitrification leads to the production of nitrite, which may be a great hazard in agro-/industrial processes and especially if marine or freshwater animals are produced within the industrial system. Due to the slow growth rates of the nitrifying bacteria, these organisms will always be in the “underdog” position to other heterotrophic organisms. The main reason for this is that the nitrifying bacteria applies the highly energy requiring process of carbon dioxide fixation by the Calvin cycle, whereas heterotrophs utilise available organic carbon in solution for its anabolism. This main metabolic constraint is followed by the further outlined growth limitations of this organism, which is not recognized in patent WO 97/49279.    1. One of the most limiting factors is the need for oxygen for nitrification. In complete nitrification 1 g of ammonia requires 4.25–4.33 grams of molecular oxygen. A rather low concentration of ammonia of 4 mg/l, thus requires an oxygen concentration of 17 mg/l, for nitrification to be complete. This oxygen concentration is not even present at water temperatures as low as 0° C., where oxygen is present at 14.6 mg/l in fresh water at normal ambient oxygen partial pressure. At normal process temperatures around 20° C., as in many indoor industrial processes, water oxygen concentrations will not exceed 9 mg/l, at which nitrification will be incomplete at ammonia concentrations above 2.65 mg/l. To achieve complete nitrification at high ammonium concentrations oxygen has to be dissolved in the water, either by explicitly adding (aeration or liquid oxygen/air) or passively by having a large contact area to the air as in trickling filters, for example.    2. Low ammonia concentrations, lower than 4 mg/l will lead to reduced nitrification rates because the Michaelis-Menten half saturation constant, which is 1–3 mgN—NH4/l, causes sub-maximum nitrification rates below levels of 4 mg ammonia/l water. Thus, in systems with low ammonia concentrations, nitrification rates are always sub optimal. Low ammonia concentrations, around 1–2 mg/l, where the need for oxygen is low, thus lead to the concomitant reduction of the nitrification rate to 25–50% of its maximum capacity with a corresponding decrease in growth.    3. As long as there is moderate concentrations of biodegradable organic matter in the water, the growth of heterotrophs by far outcompete the autotrophic nitrifying bacteria. In the present invention this problem is over come by placing the nitrification in a by-pass mode outside the main water stream, to create a highly specialized environment for the nitrifying bacteria.    4. High flow rates of water through nitrification reactor usually mean incomplete nitrification. Due to low residence time the nitrite oxidizing bacteria will not be able to oxidize all nitrite into nitrate. This is especially true in systems where high flow rates of water are applied and ammonia levels exceeding 4mg/l As a result toxic nitrite is accumulated in the system.
In summary    1. In natural conditions, oxygen levels are usually to low for nitrification to be complete, even at very low water temperatures with high oxygen solubility.    2. When ammonia levels are low, lower than 2–3 mg/l oxygen may not be limiting, but then instead, the nitrification rate becomes reduced.    3. In the main water stream the nitrifying bacteria are easily out-competed by heterotrophs, due to high organic load.    4. At high water flows incomplete nitrification will be the result from the slow growth rate of the nitrite oxidizing bacteria, compared to the water flow rate.    5. Thus in most cases either oxygen concentration or ammonia concentration is-too low, or BOD content or water flow is too high. In most cases one of these four situations are predominant in the main stream of most continuously operating water purification systems. They all result in incomplete nitrification. In WO 97/49279 the inventors themselves have provided the evidence of incomplete nitrification with reported nitrite levels as high as 15–50 mg nitrite/l for several weeks. At such levels most fish species would perish (rainbow trout has LC50 values at 0.03–0.06 mg/l). This is nowhere better displayed In WO 97/49279 , than when the nitrite levels indeed drop abruptly and are reduced to a minimum with the concomitant application of denitrification in the purification process. Thus, it is clear that the patent WO 97/49279 has hampered the nitrification capacity in at least one of the previous four conditions mentioned above.
Other similar systems, such as DE 38 27 716, have positioned the water purification bioreactors out of the mainstream water flow. In this case denitrification is placed before nitrification. This has the advantage of consuming BOD In the denitrification process before nitrification is applied. But still, water flow leading to the nitrification reactor will contain high amounts of organic material that will hamper nitrification, since no BOD-oxidising reactors are positioned in-between these two processes. Also, the water flow rate leading to the denitrification reactor can support denitrification at water flow rates far exceeding the reaction rates of the nitrifying bacteria. In addition, the purified water is funnelled back to a collection tank and being mixed with incoming non-purified water. Naturally, it should be considered a bad management practice to mix non-purified water with newly purified. In addition, the bioreactor media is are fixed beds in both cases, which contain the limitations, described earlier.
Regarding disclosures of biological phosphorous removal, U.S. Pat. No. 5,380,438 discloses processing of phosphorous containing water in anoxic and anaerobic conditions before applying aerobic phosphorous removal and nitrification in an activated sludge process. This invention has the limitation of applying nitrification in the same reactor as biological phosphorous removal. It requires competition of PAO (Phosphate Accumulating Organisms) with the nitrifying bacteria in the same reactor. It is well known that any aerobic ammonia containing sludge will develop nitrification in temperatures and pH applicable to phosphorus removal. Thus, nitrification bacteria will compete with the PAO in this type of reactor. Further, the nitrification produces nitrate that is known to inhibit the PAO process and, thus, this system is inherently sub-optimal.
Another phosphorous removal concept makes use of cyclical discharge of activated sludge or mixed liquor to three different basins to obtain anaerobic and aerobic PAO conditions. The process of U.S. Pat. No. 4,948,510 does distinguish between anaerobic and aerobic conditions. Furthermore, the competition between nitrifying bacterial and PAO accumulation in the aerobic tank is admitted, as well as the competition between heterotrophic carbon use and PAO carbon uptake, which are simultaneously applied. To solve the problems of competing nitrifying bacteria this Invention applies a rather complicated 6 (six) cycle system in three different basins. The three limitations with this system are:    1. The sludge is always more difficult to control than biofilm processes on suspended carriers, especially in combination with aerobic anaerobic processes.    2. The sludge is containing all the microorganisms, nitrifiers, denitrifiers, aerobic heterotrophs and PAOs at the same time, exposing them to cyclic changes and differential metabolic lag phases in the six purification cycles.    3. The microorganisms are forced to compete for the same space, and at times, same organic material.
Regarding greenhouse cultivation of plants, one invention defines the cultivation of water living animals with photosynthesising water living plants (WO 83/03333). The water living plants are living “on land” and are moisturised by water film according to the specification. Specifically, the disclosure points out that the water is purified by consuming the nitrogen and phosphorous therein. It is known from such trials, for instance in applying plants for water purification In aquaculture, that a plant water purification area of at least 70% of the total production plant area is needed. Thus, such a system is not efficient for the water purification itself without extensive additional water purification, unless of course the cultured plants are the main production objective and the other industrial production units are regarded as by-products (e.g. fish).
High rate closed loop industrial systems or systems for food processing, agro- or aquaculture production, with internal processing of BOD, nitrogen and phosphorous need to be cost efficient, reliable for control, and easy to operate, with high turn over rates for waste in the industrial water treatment. This is not easily obtained with activated sludge or high-pressure systems (U.S. Pat. No. 4,948,510). Among others, high-pressure systems excerpt an exceeding bioerosion compared to low-pressure systems. Furthermore, high-pressure systems also require above average capital investments.
The present invention is the starting point for an era of low energy, continuous reactor and bioreactor system with large filter area and high cost efficiency. It is a system for the biological purification of BOD, nitrogen and phosphorous for closed loop industrial systems.